Where does "Plod" originate?

[Steve Harrison]

Anyone with children will know Noddy's mate Mr. Plod. Not perhaps the most dynamic law enforcement officer of all time, although he did get a notable result when he incarcerated Sly The Goblin for TWOCing Noddy's car, seemingly without assistance from the CPS or even a court for that matter.

Anyway, does anybody know if this is the origin of this, one of the rather more affectionate terms for a police officer. Was it Enid Blyton? I suspect it was in use well before then.

[davidy]

He only nicked Sly because the light turned red!!

davidy

[big rumbly]

The dictionary definition is

"The act of moving or walking heavily and slowly.
The sound made by a heavy step. "

Refering to pounding the beat perhaps.